The present invention relates to techniques for manufacturing a semiconductor device and more particularly, to a technique which is effectively applied to a method and its apparatuses for performing accurate control over the quantity of exposure light and focus in a light exposure step.
A semiconductor device is manufactured by iteratively executing, for each layer, a step of forming a conductive or insulating film on a wafer and a lithography step of applying resist as photosensitive agent on the film, exposes and develops a circuit pattern on a reticle on the resist and then etches the film with use of the residual resist as a mask to thereby form a circuit pattern on the wafer.
As a reference technique to the present invention, a light exposure step of printing a pattern on a photosensitive agent in an exposure step will be explained in connection with FIG. 19. A product circuit pattern region 50 of a reticle 5 has a circuit pattern 51 depicted thereon, and a test pattern 52 is provided outside of the region 50. Exposure light 4001 is used to transfer the circuit pattern on a photo-sensitive agent on a wafer 1 via an exposure lens 4. For the purpose of checking to determine if the transfer circuit pattern conforms to its dimensional specifications, its dimensional inspection is carried out usually by a scanning electron microscope (SEM). The inspection is carried out by directly measuring a transfer circuit pattern 151 or by measuring a transfer test pattern 152 present outside a chip region 150. Depending on the magnitude of the measured dimensions, correction is generally carried out with the quantity of exposure light in an exposure apparatus. Automation of the light quantity correction is described, e.g., in Implementation of a Closed-loop CD and Overlay Controller for sub 0.25 μm Patterning, SPIE Vol. 3332, 1998, pp. 461-470.
Meanwhile, the causes of the dimensional fluctuations include, in addition to fluctuations of the exposure light quantity in the exposure apparatus, a focus shift. A method for correcting not only the light exposure quantity but also the focus is disclosed, e.g., in U.S. Pat. No. 6,150,664. In this method, waveform changes in an SEM are previously associated with focus shifts to find a correction for the light exposure quantity and a correction for the focus.
A recent scatterometry method for optically measuring a sectional profile of a transfer circuit pattern is disclosed, e.g., in Specular Spectroscopic Scatterometry in DUV Lithography, SPIE Vol. 3677, 1999, pp. 159-168.
Explanation will now be made as to the arrangement of a scatterometry measuring apparatus by referring to FIGS. 20A and 20B. FIG. 20A shows a specular spectroscopic scatterometry measurement apparatus. White color light 2001 emitted from a white color light source 201 is irradiated on a repeated pattern 12 on a substrate 11 so that regularly-reflected light is spectrally separated by a diffraction grating 202 and detected by a sensor 203. Unlike the spectral type measurement apparatus for obtaining an optical intensity signal for wavelength, there is an incident angle change type which obtains a light intensity signal or signature for incident angle. In a measurement apparatus shown in FIG. 20B, an angle 9 of incident light 2002 is varied to irradiate the light on an object and to detect a regularly-reflected light 2003.
Explanation will next be made as to a method for processing a light intensity signal obtained in the above measurement apparatus with reference to FIG. 21. A light intensity signal 21 called a signature, when obtained in the measurement apparatus of FIG. 20A, indicates a light intensity change to wavelength. The signature varies with the sectional profile of the repeated pattern 12. Thus, signatures for various sectional profiles are previously found by wave optics simulation and stored in a library. For example, the sectional profiles are modeled in rectangle depending on a bottom line width L, film thickness D and taper angle α of the repeated pattern 12 to perform simulations over the signatures. The signature 21 is compared with the signatures in the library to find a coincided signature and to use a sectional profile providing the coincided signature, that is, a line width L1, film thickness D1 and taper angle α1 as measured values.
Scatterometry based on optical measurement is advantageous over the SEM because the reaction of the photosensitive agent may cause the line width to change during irradiation of electron beam in the SEM. Further, the scatterometry can measure in the atmosphere and, unlike the SEM requiring a time for evacuation, can measure at a higher speed.
As has been explained above, the scatterometry has a merit over the SEM in measurement of the circuit pattern sectional profile. However, the scatterometry requires previous calculation of a large number of waveforms and thus requires high-speed optical simulation. To this end, a calculation method called “Rigorous Couple Wave Analysis” and disclosed in Diffraction Analysis of Dielectric Surface-relief Gratings, J. Opt. Soc. Am., Vol. 72, No. 10, 1982, is employed. This is a method for approximating a pattern section as a plurality of rectangular layers, regarding the rectangular layers as endlessly-continuous diffraction gratings of an identical pitch and duty, and joining boundaries therebetween to determine a coefficient for the solution of a wave equation. When this method is compared with a finite element method or the like as another solution of the wave equation, waveform calculation can be carried out at a very high speed.